One of the basic principles of weed control is that small weeds are easier to control than large weeds.

However, spraying too early, such as trying to control less than 2 leaf annual ryegrass, phalaris or many other small, upright grass weeds., can reduce control because these plants present a very small, hard-to-hit target.

Control can be greatly improved by delaying application to the 2-to-3-leaf stage.

This also allows time for more weeds to emerge, so a larger proportion of the weed population will be controlled with one herbicide application.

However, for many resistant populations, delaying application past the commencement of tillering will often result in reduced control

Some benefits of spraying small weeds include:

• young, small weeds are often more susceptible to herbicides – for example, flaxleaf fleabane (Conyza bonariensis) is relatively easy to control with label rates of herbicides at up to 5-centimetre diameter rosettes, but after that effective herbicide control tends to become highly variable;

• reducing the risk of yield losses to competition between weeds and crops;

• difficulty obtaining uniform coverage of large weeds;

• older plants can have thicker or hairier cuticles, which slow or prevent herbicide absorption – for example fleabane;

• older plants can be better at breaking down herbicides before they get to the target site – a number of studies have shown that herbicide resistance is stronger in older plants; and

• delaying initial applications until weeds are taller than 10cm may complicate the implementation of secondary control strategies if the weeds survive the first application through environmental stress, poor application or herbicide resistance.

Notes

Note that not all small weeds are easy to control as they might be small due to environmental stress.

Soil-applied herbicides used at crop-selective rates must be applied to the soil before weeds have established because they are most effective on germinating seeds and emerging seedlings. Larger weeds can grow their roots below the herbicide band or are able to out-grow the effects of the herbicide at these rates. Higher herbicide rates that are used for total vegetation control will control established annual weeds, however they still give poor control of perennial weeds that use vegetative reproduction such as tubers and rhizomes.

Time of day

The effectiveness of some herbicides can be influenced by the time of day the herbicides are applied. Much of this effect goes back to how the herbicides are absorbed and translocated within the plant, as well as their mode of action. Refer to section 2.5 in this module, ‘Impact of environmental factors on herbicide activity’.

The bipyridyl herbicides paraquat and diquat are the most affected by time-of-day application. Due to their rapid activation in the presence of sunlight, applying bipyridyl herbicides late in the afternoon or after dark increases the time between absorption and activation. This gives the herbicide more time to translocate before being activated by sunlight.

There is some evidence that glyphosate is more reliable when applied during daylight hours when conditions are good for leaf uptake, that is Delta T conditions less than approximately 6-8. 'Summer' applications of glyphosate under hot and low humidity conditions (Delta T above about 10) during, or for the 4-6 hours post application, typically reduce glyphosate performance. Most of these environmental effects can be minimised by using a robust rate of herbicide and applying under the best possible conditions.

Notes

Delta T is the difference between wet and dry bulb temperature. Higher Delta T values indicate a greater potential for rapid droplet evaporation.